The present invention relates to the field of optics.
Synthesis of three-dimensional light beams is an essential part of scanning devices, plotters, light pointers, optical communication modules and super-resolution devices. Today, light beams are generated by use of computer-generated holograms, and by fabrication of diffractive optical elements.
Known methods for synthesizing light beams suffer from divergence of shape over large distances. As a result, optical scanners are limited to short distances in their operation. This has practical disadvantages. For example, it implies that optical bar code readers must be located sufficiently close to the bar code being read in order to read it properly. If a teller does not move a bar code reader close enough to the product being read, the reader does not properly register the bar code that is on the product. Although the teller can be far from the scanner, the scanning region itself must be sufficiently short in order to properly read a bar code.
Several approaches for generating xe2x80x9cnon-diffractingxe2x80x9d beams, i.e., beams which preserve their spatial properties while propagating along a transverse axis, have been developed over the past 20 years. One such approach uses an exact non-diffractive solution of the scalar wave equation, referred to as a xe2x80x9cBessel beam.xe2x80x9d However, the Bessel beam is ideal in that it is unbounded and requires an infinite amount of energy for its generation, and, as such, its use has limited practical value. A reference for this approach is J. During, Exact solutions of nondiffracting beams, J. Optical Society of America A 4, 1987, pages 651-654, the contents of which are hereby incorporated by reference.
Another such approach uses plane waves, which are also ideal non-diffractive beams. However, plane waves are also unbounded and require an infinite amount of energy for their generation, and, as such, their use has limited practical value.
Although the use of the abovementioned ideal non-diffracting beams is impractical, it is possible to generate approximations to non-diffractive beams by means of diffractive optical elements. A reference for this is A. Vasara, J. Turunen and A. T. Friberg, Realization of general non-diffracting beams with computer generated holograms, J. Optical Society of America A 6, 1989, pages 1748-1754, the contents of which are hereby incorporated by reference.
Yet another approach to generating non-diffracting beams is to multiply the Bessel beam by a Gaussian profile, producing what is referred to as a xe2x80x9cGauss-Bessel beam.xe2x80x9d The Gauss-Bessel beam carries a finite amount of energy, and despite its diffraction sensitivity due to the Gaussian profile, its diffraction spread is small as compared with typical Gaussian beams. A reference for this approach is F. Gori, G. Guattari and C. Padovani, Bessel-Gauss beams, Optical Communications 64, 1987, page 491, the contents of which are hereby incorporated by reference.
A further approach to generating non-diffracting beams uses an iterative technique referred to as Projection onto Constraint Sets (POCS). POCS operates by performing free space propagation of an incoming plane wave through a diffractive light filter, and forward through a series of parallel planes along an axis transverse thereto. At each plane of the series of parallel planes, the light distribution obtained by free space propagation is modified according to desired constraints, and then the modified light distribution is propagated through free space to a next parallel plane, until the propagation reaches the last of the parallel planes. After the light distribution is propagated to the last of the parallel planes, it is then propagated along the transverse axis back to the light filter, using backward free space propagation. Successive iterations of the POCS algorithm then further propagate the resulting light distribution forward through each of the parallel planes, and then backward back to the filter, as described hereinabove. The sequence of light distributions that is generated in successive iterations typically tends to stabilize at a single distribution, and the POCS iterations are terminated when a prescribed convergence criterion for the sequence of generated light distributions is met.
References for the iterative POCS approach are J. Rosen, Synthesis of nondiffracting beams in free space, Optical Letters 19, 1994, pages 369-371, and R. Piestun, B. Spektor and J. Shamir, Wave fields in three dimensions: analysis and synthesis, J. Optical Society of America A 13, 1996, page 1837, the contents of both of which are hereby incorporated by reference.
Many approaches to generating non-diffracting beams involve complex amplitude light distribution functions. Such light distribution functions can be generated using a single phase-only filter, as described in D. Mendlovic, G. Shabtay, U. Levy, Z. Zalevsky and E. Marom, Encoding techniques for the design of zero order (on Axis) Fraunhofer computer generated holograms, Applied Optics 36, 1997, pages 8427-8434, the contents of which are hereby incorporated by reference. Alternatively, complex amplitude light distributions can also be generated using two phase-only filters separated by a propagation distance, as described in D. Mendlovic, Z. Zalevsky, G. Shabtay and E. Marom, High efficiency arbitrary array generator, Applied Optics 35, 1986, pages 6875-6880, the contents of which are hereby incorporated by reference. Use of two phase-only filters typically generates complex amplitude light distributions with higher efficiency, as compared with use of a single phase-only filter. The above approach of using two phase-only filters is based on application of an algorithm of Gerchberg-Saxton to a Fresnel domain, as described in Z. Zalevsky, D. Mendlovic and A. W. Lohmann, Gerchberg-Saxton algorithm applied in the fractional Fourier or the Fresnel domain, Optical Letters 21, 1996, pages 842-844, the contents of which are hereby incorporated by reference.
The present invention provides methods and systems for synthesizing light beams with non-diffractive characteristics. Specifically, it can be used to synthesize elongate beams useful in optical scanning devices that operate over larger distance ranges than conventional beams.
The present invention provides both a direct analytical approach and an indirect iterative approach for approximating desired light beams. The analytical approach is guaranteed to produce optimal light beams in a mean square error sense, as described hereinbelow.
The present invention has advantageous utility when used in conjunction with optical scanning devices. It can be used to synthesize elongate beams that operate over larger distance ranges than conventional beams. It can also be used to synthesize beams within prescribed regions of interest.
The present invention also has advantageous utility when used for illuminating targets at a plurality of distances. By controlling the three-dimensional characteristics of a light beam, the present invention can be used to generated beams with elongated focus and point targets at a plurality of locations.
Based on an analogy between diffractive free space propagation and dispersion within linear fibers, the present invention also has advantageous utility when used in conjunction with optical communication systems. It can be used to synthesize time-varying one-dimensional beams in optical fibers containing multiple exit points where information may be extracted.
There is thus provided in accordance with a preferred embodiment of the present invention a method for synthesizing a desired light beam including calculating a two-dimensional light filter for an optical element, the two-dimensional light filter being such that the optical element produces under free space propagation, in response to illumination thereof, a three-dimensional light distribution that approximates the light distribution of the desired light beam, and illuminating the optical element.
There is further provided in accordance with a preferred embodiment of the present invention a method for synthesizing a desired light beam including calculating a plurality of phase-only light filters for a respective plurality of optical elements, the plurality of phase-only light filters being such that the plurality of optical elements when mounted in series produce under free space propagation, in response to illumination thereof, a three-dimensional light distribution that approximates the light distribution of the desired light beam, and illuminating the plurality of optical elements.
There is still further provided in accordance with a preferred embodiment of the present invention a system for synthesizing a desired light beam including a processor calculating a two-dimensional light filter for an optical element, the two-dimensional light filter being such that the optical element produces under free space propagation, in response to illumination thereof, a three-dimensional light distribution that approximates the light distribution of the desired light beam, and an optical element filtering incoming light according to the two-dimensional light filter.
There is additionally provided in accordance with a preferred embodiment of the present invention a system for synthesizing a desired light beam including a processor calculating a two-dimensional light filter for an optical element, the two-dimensional light filter being such that the optical element produces under free space propagation, in response to illumination thereof, a three-dimensional light distribution that approximates the light distribution of the desired light beam, and a plurality of optical elements with phase-only light filters mounted in series which, when used in combination, filter incoming light according to the two-dimensional light filter.
There is further provided in accordance with a preferred embodiment of the present invention an elongated focused light beam for use within an optical scanner, made by the process of calculating a two-dimensional light filter for an optical element, the two-dimensional light filter being such that the optical element produces under free space propagation, in response to illumination thereof, a three-dimensional light distribution that approximates the light distribution of a desired light beam, and illuminating the optical element.
There is still further provided in accordance with a preferred embodiment of the present invention an elongated focused light beam for use within an optical scanner, made by the process of calculating a plurality of phase-only light filters for a respective plurality of optical elements, the plurality of phase-only light filters being such that the plurality of optical elements when mounted in series produce under free space propagation, in response to illumination thereof, a three-dimensional light distribution that approximates the light distribution of a desired light beam, and illuminating the plurality of optical elements.
There is additionally provided in accordance with a preferred embodiment of the present invention an article of manufacture including one or more computer readable media that embody a program of instructions for synthesizing a desired light beam, wherein the program of instructions, when executed by a processor, causes the processor to calculate a two-dimensional light filter for an optical element, the two-dimensional light filter being such that the optical element produces under free space propagation, in response to illumination thereof, a three-dimensional light distribution that approximates the light distribution of the desired light beam.
There is further provided in accordance with a preferred embodiment of the present invention a method for synthesizing a desired time-varying light distribution propagating along an optical fiber, including the steps of calculating a time-dependent light source which produces, under propagation along an optical fiber, a time-varying light distribution that best approximates the desired time-varying light distribution with respect to a prescribed error metric, as compared with time-varying light distributions produced by other time-dependent light sources, and propagating the time-dependent light source along the optical fiber.
There is still further provided in accordance with a preferred embodiment of the present invention a method for synthesizing a desired time-varying light distribution propagating along an optical fiber, including the steps of choosing a candidate time-dependent light source for propagation along an optical fiber, deriving a plurality of forward time-dependent light sources at a respective plurality of locations along the optical fiber, by applying forward dispersion to the candidate time-dependent light source, modifying the plurality of forward time-dependent light sources, further deriving a plurality of backward time-dependent light sources by applying backward dispersion to each of the plurality of modified forward time-dependent light sources, producing a modified candidate time-dependent light source by taking an average of the plurality of backward time-dependent light sources, and propagating the modified candidate time-dependent light source along the optical fiber.
There is additionally provided in accordance with a preferred embodiment of the present invention a system for synthesizing a desired time-varying light distribution propagating along an optical fiber, including an acousto-optic modulator creating a time-dependent light source which produces, under propagation along an optical fiber, a time-varying light distribution that best approximates the desired time-varying light distribution with respect to a prescribed error metric, as compared with time-varying light distributions produced by other time-dependent light sources, and an optical fiber propagating the time-dependent light source.
There is further provided in accordance with a preferred embodiment of the present invention a system for synthesizing a desired time-varying light distribution propagating along an optical fiber, including a dispersion unit applying forward dispersion to a candidate time-dependent light source, producing a plurality of forward time-dependent light sources at a respective plurality of locations along the optical fiber, and applying backward dispersion to modified forward time-dependent light sources, producing a plurality of backward time-dependent light sources, a modification unit modifying the plurality of forward time-dependent light sources, producing the modified forward time-dependent light sources, an averaging unit generating an average of the plurality of backward time-dependent light sources, producing a modified candidate time-dependent light source, and an optical fiber propagating the modified candidate time-dependent light source.